DIY Spectro

UPDATE: There is a newer, more automated version of this project here, and an FAQ section here.

For the last while I have been concentrating on a project: developing an easily built spectrophotometer for low budget and DIY laboratories.

At the subatomic level, light is made up of entities called photons. Photons are electromagnetic vibrations; the speed at which they vibrate (in vibrations per second) determines the color of the light. Red light has a relatively slow vibrational frequency, while purple light has a faster one. The frequency also determines the energy that the the photon has: the faster a photon vibrates, the more energy it has. A photon of violet light has more energy than a photon of red light.

White light, like that from a halogen lamp, contains photons of a lot of different frequencies. However, you can use a prism or a diffraction grating to break that light up into its component colors, to get the familiar rainbow:

The spectrum of a halogen lamp

The above image is from a spectroscope I built during development. It’s made from bamboo, duct tape, pieces of beer cans, and an old CD: the perfect combination of steampunk, cyberpunk, and drunkpunk.

Compare it to the spectrum of the halogen lamp. Notice the dark bands, especially in the yellow region? Those are absorption bands. Chemicals- like the coating on the inside of a frosted lightbulb- will often absorb light at specific frequencies. When those frequencies get absorbed, their corresponding colors are missing from the rainbow*. This is useful because the specific absorption pattern can act as a molecular fingerprint; by looking at a chemical’s absorption pattern, you can identify it.

A spectrophotometer is like the spectroscope I built, but it replaces the human eye with an electronic measuring device. Here’s the current setup:

putting the "spectro" in spectrophotometer

The light source is an LED flashlight. The light shines through the sample (in this case a vial of chlorophyll) and gets broken up by a diffraction grating. This produces a spectrum which gets projected….

… onto the photosensor. I pulled the sensor out of an automatic night light. It is mounted on a stand, which is taped to a TI89 which is taped to the table- so I can slide the sensor back and forth along the spectrum to get readings at different frequencies. I measured the frequency of light hitting the detector by noting where its shadow falls on the ruler in the background. The resistance of the sensor changes depending on how much light falls on it (which is an indication of how much light gets absorbed by the sample); I measure this with a multimeter.

putting the "photometer" in spectrophotometer

How well does this setup work? Here’s the absorbance spectrum I measured for that sample of chlorophyll.

The absorbance spectrum of cholorphyll, as measured by the above setup.

Chlorophyll absorbs most strongly in the red and blue parts of the spectrum, and absorbs weakest in the green region. That’s why it (and hence plants) are green- its the only light left. Sure, the spectrum I measured isn’t as clean as a more official spectrum but you can still see the parts of sunlight that plants turn into noms- pretty good for being made out of toilet paper rolls!

Next step: Servo motors and computer control for great win!

*This is assuming that the colors were there to begin with. Halogen and incandescant bulbs both create light by using electricity to make a bit of material so hot that it glows, and all objects that glow from heat emit light at all frequencies at some brightness or other– so the assumption is valid in this case. However, there are a lot of light sources which only emit a handful of frequencies- when I look at my computer screen through the spectroscope, the light is mostly a few bands in the red, blue and green regions. You can use these emission patterns as chemical fingerprints just as you can use absorption patterns. Suffice to say, I have been swaggering around at night with my scope, examining street lights and neon signs, and generally perturbing passersby.

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15 Comments:

Neat project, and basic science at the same time! I think after I got a basic machine up and running well enough to be portable, I’d also be checking out various light sources (I have strong interests in general photography and astronomy).

It looks like you’re already perusing the next step – building an automated device with positioning motors and a movable sensor. I’d like to suggest an alternative – use a linear optical sensor (CCD) as you might find in a flatbed scanner, fax machine, or many handheld barcode scanners. This removes all the mechanical problems, as all the mechanical bits can be glued in place and the resulting device calibrated in software.

The trick would be to ensure that the sensor has a mostly even luminous response across the optical spectrum. This is may also be an issue with the “nightlight” sensor issue that you’ve already probably already seen (and perhaps somewhat compensated for, within the limits of the sensor.)

Silicon-based senors (photo-transistor, photo-diode, and CCD devices) tend to have broad, flat, and fast responses at small points. CdS photosensors tend to be relatively slow response (and I think the spectral response is inferior to silicon) as well have having a large sensing area.

just a thought, but the linear CCD from a broken all-in-one printer could work here.
plus its already set up to detect a wide range of wavelengths as long as its one of the units that uses a RGB LED to select the colour.

Good call! CCDs are everywhere, apparently. Do you know of other places to scavenge them?

I have a small digital camera unit that I pulled out of a broken iPhone that I found in a parking lot- once I figure out how to interface with it, it’s going on the eyehole of the spectroscope. The goal for the spectrophotometer is to optimize cost and ease of assembly first- you don’t necessarily need to be able to read the hydrogen lines in the solar spectrum in order to do useful colorimetry, for example. But imaging tech has gotten so cheap that it doesn’t seem like that great of a leap to relatively sophisticated, low budget spectroscopy (atomic emission, etc.)

I should have commented a week ago – Bravo! I haven’t yet developed the part of my brain to build such devices, but this is really amazing. Keep it up! I’m all for DIY biology – I hope to get involved in it at some point.

Actually, for a lot of uses, you don’t even need the whole spectrum. I run a lab where we quantify the amount of plankton in a sample by the ratio of absorbance at just 2 wavelengths… so mount 2 resistors at those points and it might be sensitive enough for some quantitative work.

Similarly, I use a spec at 3 wavelengths to measure the pH of a sample by dye change… for big shifts in pH, you can see the color change, and with 3 photoresistors and 3 multimeters, you could just write the numbers down.

I’m not throwing away my CCD spec yet, but this could be a neat teaching project. Wish I had some time to play with comparisons against more expensive lab equipment to see how it does.

It’s not pictured but there is a slit in the diffraction grating. It’s also a transmission grating, not the CD chip I use for the scope.

I hadn’t realized the importance of the slit until I started actually putting things together. I also hadn’t realized that the *orientation* of the slit also matters. If you don’t do it correctly, the bands overlap and create an area of white in the center with red and blue on the edges.

Slit size seems to control the resolution of the spectrum- the smaller the slit, the narrower band you can look at. On the other hand, the smaller the slit, the less light passes through. One of the reasons I’m developing the project in the way I am is to get a feel for the complexities and engineering tradeoffs before I jump into a high accuracy, high precision arena.

The spectra I show above were made by pointing a digital camera; these are too low quality* to do precision analytical work, but even they can make general claims about absorption and emission patterns. The human eye is a lot more more sensitive- a computer screen alone showed up too dimly to really pick it up well. A highly tuned setup could definitely work well though. I have a small cellphone camera I found in a parking lot; as soon as a figure out how to interface with it….

the only problem with a digital camera is that it’s fairly expensive, especially for people in the developing world. It might also provide more functionality than you need- for doing colorimetry, for example, you only need to be able to look at absorption at a given frequency. A motor, controller, and photoresistor might meet that demand for cheaper.

*HOW I TURN OFF THE AUTOFOCUS ON THIS DAGNABBIT NEWFOUNGLED TECHNOOLOGY

If you really want to get good readings hack a line scanner from a disused multifubnction printer copier scanner fax. place this in the scan path. Set the scan programme to generate a B/W tif file open this and you will get digital values on intensity

[…] can you make with a toilet paper roll, duct tape, and a graphing calculator? A stand for your homemade spectrometer. This is neither as pretty nor as accurate as a precision scientific instrument, but that […]

[…] can you make with a toilet paper roll, duct tape, and a graphing calculator? A stand for your homemade spectrometer. This is neither as pretty nor as accurate as a precision scientific instrument, but that […]